1. Field of the Invention
The present invention relates to an image processing system and an image processing method, and particularly, to an image processing system and an image processing method in which an original sheet is scanned to obtain a set of image data, and a background on the original sheet is removed from the set of image data by setting up a threshold for the removal.
2. Description of Related Arts
In the copying by printing, an image scanner scans various types of sheets as an original. Some of sheets used as an original, including a newspaper, a recycled paper, and a colored paper, for instance, have relatively high background densities. Scanning such an original sheet requires a process of removing a background, as known in the art. This process serves for clarification of letters on a copied sheet, permitting reduction of ink or toner consumption in printing.
The removal of background generally includes: estimating a background density of original sheet after an image processing; determining a threshold depending on the density; and defining a set of pixels of values not over the threshold, as a background to be removed from a frame of image data, as a series of processes uneasy to execute. As a technique to determine ‘a threshold for background removal’ (referred herein to as ‘a background removal threshold’), among three patent documents listed below, the patent document 1 (Japanese Patent Application Laid-Open Publication No. 4-313774) has disclosed: using a set of image data scanned in a pre-scan of an original sheet, for preparing a histogram within a range of background density based on the set of image data of the original sheet, to thereby determine a background removal threshold; and implementing a removal of background from a set of image data scanned in a main scan of the original sheet. The patent document 2 (Japanese Patent Application Laid-Open Publication No. 2000-101839) has disclosed determining a background removal threshold by one page of image data. The patent document 3 (Japanese Patent Application Laid-Open Publication No. 7-303188) has disclosed a method of using a histogram to determine a background removal threshold, including a dispersion calculation for determining how far peaks are spaced from each other in the histogram, to thereby determine a background removal threshold.